Control methods and control arrangements are known which receive their control variable from a gas probe mounted in the exhaust-gas system of the engine. In this way, a specific air/fuel ratio of the air/fuel mixture supplied to the engine is maintained. The gas probes can, for example, be a .lambda.-probe or an HC probe. This control is, in general, superposed on a mixture control arrangement with which the composition of the air/fuel mixture is coarsely precontrolled. The precondition for a trouble free operation of such a control unit is that the gas probe operates free of trouble.
According to the regulations of California as well in accordance with United States Federal regulations, the function of all exhaust-gas relevant components of an internal combustion engine, such as injection systems, catalatic converters and the like, must be monitored during the operation utilizing on-board equipment (on-board diagnosis OBD). The operational readiness for most known gas probes is ensured only at a specific operating temperature. These gas probes include, for example, .lambda.-probes or HC probes.
For the above reason, probe heaters are provided in such gas probes of the Nernst type. The probe heaters additionally heat the probes during the cold-start and warm-up phase of the engine so that the required operating temperature is reached earlier. The function of the exhaust-gas probe (that is, the function of the Nernst cell as well as the function of the probe heater) are exhaust-gas relevant quantities. For this reason, the monitoring thereof is required in the context of the on-board diagnosis.
U.S. Pat. No. 5,090,387 discloses, for example, a method and an arrangement for checking the operability of an exhaust-gas probe heater and its feed-line system. This method and arrangement permit monitoring the probe heater without additional sensors or lines.
It is problematic with this method and arrangement that only the probe heater can be monitored but not the operation of the probe itself.
U.S. Pat. No. 5,311,138 discloses an arrangement for monitoring the operation of the following: an electric/electronic switch (output stage), a consumer connected to the switch; the drive and the connecting lines of the switch, which, for example, can be utilized also for monitoring the probe heater. However, this arrangement, too, only permits monitoring the probe heater but does not permit monitoring the actual probe itself.